1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and particularly, to the apparatus having a particulate filter, such as a diesel particulate filter, hereinafter referred to as DPF, which traps particulates, also referred to as particulate matter, in exhaust gases of the internal combustion engine.
2. Description of the Related Art
A technique wherein a DPF is provided, which traps particulates in exhaust gases in the exhaust system of a diesel internal combustion engine and reduces an emission amount of the particulates, is widely used. Since there is a limit in the amount of particulates that can be trapped in the DPF, a regeneration process is timely performed wherein the particulates accumulated in the DPF are burned. The regeneration process is performed, for example, by executing a fuel injection during the exhaust stroke (hereinafter referred to as “post injection”) wherein unburnt fuel is supplied to the catalytic converter disposed upstream of the DPF and the fuel in the catalytic converter is burned, thereby raising the exhaust gas temperature.
It has been confirmed that execution of the post injection process gradually dilutes the lubricating oil in the cylinder. If dilution of the lubricating oil is allowed to continue unchecked, the engine cannot operate normally. Accordingly, it is necessary to avoid, or at least suppress, excessive dilution of the lubricating oil.
Japanese Patent Laid-open No. 2003-322044 (JP '044) discloses a technique for suppressing dilution of the lubricating oil in a gasoline internal combustion engine in which fuel is directly injected into the cylinders.
According to the method taught in JP '044, a dilution counter begins counting during the cold start of the engine in which the lubricating oil dilution is likely to occur. If the dilution counter reaches a value equal to or greater than a predetermined value, a fuel injection timing of the engine is changed in the advancing direction.
However, in the regeneration process of the DPF mounted in a diesel internal combustion engine, if the execution timing of the post injection process is greatly advanced or advanced too much, the fuel injected during the post injection process burns within the cylinder which results in unintended increase in the output torque. Further, if the advanced amount of the post injection timing is too small or otherwise insufficient, advancing the post injection timing is not an effective manner for suppressing dilution of the lubricating oil.